Powerful Muslim CBs allow fast expansion, but large realms often fail due to dynastic decadence. Most variants of Islam can have a caliph, a secular (and thus playable) religious head.

Iqta government replaces both Feudal and Theocracy governments, allowing rulers to hold both castles and mosques without penalty. Iqta rulers can freely revoke duchies, and use Open succession to effectively designate heirs.

Characters can convert to another Dharmic religion if they control any province with the new religion. Provinces convert very slowly, but this is okay because there is no revolt risk. No religious heads.

Variants:

Hindu: A huge holy order and +30% morale. Hindus are the only Indians who can raid and wage duchy-level holy wars. They are hindered by strict adherence to the caste system, and by lack of heir designation.

1Religion DLC is not required to play as a nomad; Horse Lords unlocks nomadic rulers of all religions. However, players must convert to a playable religion before they can settle down to a non-nomadic government type.

2All religions support concubinage when tribal or nomadic. For Muslims, this replaces polygamy for rulers.

If characters are of different religions, they'll get a large opinion penalty towards one another:

Name

Criteria

Opinion

Example

Religious Differences

Different religions of the same religious groups

-10

Catholic vs Orthodox

Heretic

Different religion within the same religious sub-group

-25

Catholic vs a Catholic Heresy

Infidel

Different religion group

-20

Catholic vs Sunni

Zealous

Anyone of different religion

-25

Stacks with standard penalty

This penalty, apart from Zealous, is reduced by the Tolerance technology in the capital of each character, -6.25% per technology level. Additionally, a character who is sympathetic towards another religious group has no Infidel penalty towards or from characters following a religion of that group.

Religion affects how Crown laws are applied. This is particularly important in regions which see frequent religious conflict (Iberia, Eastern Europe and the Near East), or are simply religiously diverse (India). Independent dukes and counts are considered to have the No Crown Authority Law, if the king of the de jure kingdom which they belong to is of another religion.

AI rulers will also never accept diplomatic vassalization if they follow a different religion from the potential liege's.

Religion also provides various Casus Belli, most notably holy war and Crusades, though this varies from religion to religion - both are unavailable to unreformed pagans for example. Sword of Islam also introduced two new Casus Belli specifically for Muslims.

Religion also affects what powers the religious head has; unreformed Pagan religions for example have no Religious Head, while a Christian Religious head can excommunicate characters, which is impossible for Muslims and Pagans.

Pagan religions may gain a religious head and a Great Holy War (Crusade CB) if they reform. They then become the "Reformed" version. The version without a religious head and Great Holy War becomes a heresy and is called the "Old" version of the religion.

Since 2.4.1 when a barony that holds a holy site is destroyed, the province holds it instead, but the value of the province for moral authority is reduced by half. But if you build a temple in the province, it will become the holy site. This was to prevent the game from crashing when hordes raze a holy site.

Because a majority of Tengri land belong to hordes, they have no temples in a majority of their holy sites, forcing the player to build them.

The Hellenic and (generic) Pagan religions appear for characters in the history files, representing (for Hellenism) the pre-Christian Roman emperors and (for Paganism) pre-Christian Irish and pre-Islamic Arabs, among others. The rare "Spawn of Satan" event can lead to the creation of 3 generic pagan witches, but otherwise they cannot normally appear in-game without the console.

Catholicism and all Catholic Heresies become Orthodox heresies if the great Schism is mended. This does not affect the Miaphysite or Nestorian denominations.

All religions with a parent religion are heresies.
With Sons of Abraham, if a heresy becomes more prevalent than the parent religion (5 provinces more), it will become the new orthodoxy, and the parent religion will become a heresy. All holy orders of the parent religion switch over to the former heresy when this happens. Note that the Indian religions do not have heresies.

When a pagan religion is reformed, anyone following it gets an event to decide whether to accept the reformation. If they don't, they become the "Old" version, which is considered a heresy. The reformed version keeps the name of the pre-reformation religion.

Likewise, if an Orthodox ruler mends the Great Schism, Catholic rulers get an event to decide whether to convert to Orthodoxy (or the "heresy"). Either way, Catholicism is thereafter treated as an Orthodox heresy, and all the Catholic heresies become Orthodox heresies instead. Orthodox Heresies can also mend the Great Schism if they manage to replace Orthodoxy as the parent religion.